Delco starter overhaul.

The FAC 241 and the later 240/241 265 etc are completely different engines, nothing interchanges between the small six and the medium six.
The FAB, FAC, BLD and BD 282 and 308 are closely related, and some parts interchange and some don not.
Complete engines in the same family will often bolt in place of each other, although the later BD 282 and 308 relocated the oil filter and it hits the steering gear if you try and put it in the K series trucks. The oil filter adaptors do NOT interchange so you must use the one the engine came with.
Things like manifolds and heads are different the later ones have bigger valves and ports, so you can't fit on an early engine.
The engine series that started in the late 20-s early 30's that was the FAB BLD (BD 282 and 308 only) lasted all the way up to about 1970, on paper at least. When IHC came out with SV V-8 most trucks got the V8 and the medium six is rare to find after the mid 50's, The SV started at 266 then 304 and ended up 345 and 392. It covered the same displacement area that the 282 and 308 did. There are few like me that would have prefered the inline over the V8 and the six didn't sell well after the V8 was available.
Just to be very clear, the small six that was in the pick-ups and lighter medium trucks starting in 1950 on are completely different than the larger medium six.

I have not been out to the shop as we have snow but I did find a link that the D35 had a FAB241 and the KB6 had the improved FAC241. I could not seem to copy the link. I will confirm what I have.

See if you can open this.
https://tractors.fandom.com/wiki/Intern ... BS-6_Truck

Yeah it opened, I wouldn't take that as gospel, for one, they got the displacement of the FAC wrong, 241 and 259.
The FAB was fairly early dating back to the early 30's if not before, FAB's I've seen have the gear driven genny of the left side of the engine and the fuel pump on the right. The FAC's I've seen the gear driven genny is gone and the fuel pump is on the left.
When IHC made major changes to an engine, they always seamed to change the name or model number. It is definitely true when they went from wet liner to dry (FAC to BLD) and again when they went to bores in the block and full flow oil filtering (BLD to BD). Given that, I would expect that a major change like a redesign of the timing cover, moving the fuel pump and distributor would be logical that there would be a name change as well (FAB to FAC). True, I haven't seen it written anywhere, and I could be wrong, but from my time messing around with this engine family, I have gleaned a little insight into IHC engine names.

Opening up a few more of my old starters, I am finding a common theme to the problems. The CE of the armature rides directly in the cast rear cover. The shaft is lubed by a oiler and oil wick in the back cover.
While it may have been common practice "back in the day" and they may have got the service they needed when vehicles were "on the lube rack" regularly. I am finding that mine are dry when disassembled, and not because they were overlooked (although if 100% honest, it didn't fill them monthly either). The problem is dirt clogging the wick preventing oil in the cup from lubing the shaft. Keeping the cup full is of little use if the oil doesn't make it to the shaft.
The DE bushing is either dimpled or oilite and is a 9/16" ID x 5/8" OD bushing. They can hold grease or oil to lube the shaft.
My Air starters on my big trucks use a unitized needle bearing, for the DE and ball bearing for the back end and give almost permanent service, I have not replaced one and only have to re grease the DE when the starter is out for some reason. The rest are lube by a diesel injector into the air stream
So what can be done for these old 6 volt starters? The CE cover has a expansion plug (11/16") and the bore is 9/16". I am going to opt for a 5/8" reamer to ream so it can take the same bushing that is used in the DE.
I have seen some aftermarket CE covers that look like that is what they did as well. One could even install a grease nipple in place of the oil cup and wick to re-lube as long as care is taken not to over grease and get grease on the commutator. An oilite bushing and keeping the original oil cup and wick would also work well.
It is easy enough to pop the expansion plug out, and pass a 5/8" reamer through then press in a bushing and ream to 9/16".
On the BLD's the starter is up under the manifolds and not easy to reach and re lube, which is why I would guess many are so neglected.
It is very hard to "wash out" the oil wick of caked in dirt, so just being sure the cup is full is not enough to be sure the starter is serviced.
If you have good cables, and no "hot" connections, but still have slow cranking, I would pull and inspect the CE of the starter.

Curiosity got the better of me, I went and looked at the factory lube chart. Oil the starter with a few drops of engine oil every 500-1000 miles! Yeah, I bet that got done!

Thanks for the additional info. I am not sure I would want to oil it every 500 miles anyway but I doubt almost anyone did that!

I went out and checked the engine in my D35 today and it is a FAB241. That is what I thought I remembered it being. From what I remember the FAC is a improved version. I think there is a Wikipedia site that outlines some of the differences. Not that those sites are accurate.

Does it have a distributor over the valve cover or a gear drive genny/distributor on the timing cover?
I wonder when the change was made, and if some older units were "retro fitted" with the later design when the generator quit? By the mid 30's most of the gear driven generators had gone, but I have a 50's 6-71 Detroit (GM Diesel at that time) with a gear driven genny.
Also, which side is the fuel pump on?

Distributor over valve cover. Generator is a separate unit like a normal mounter generator.

The original design had the gear driven generator on the left side (driven from the cam gear) and it precluded having the fuel pump driven direct from the cam. There was a push rod system and the fuel pump was mounted on the right side where the generator was on later engine. Push rod ran across the block from left to right.
In fact, if you remove the generator mount on my BLD's you will find a rough casting opening (not machined out) for a fuel pump. They didn't change the mold, just made a plate to go over the hole!
IIRC by the time of the BD 282 and 308, it was gone from the mold but there were many changes to the mold for the later engines.
It shows when the FA series was designed, there wasn't a lot of thoughts about what was to come. The FB series they did a better job, and it lasted with little change right up until the 70's when diesel and emissions killed off the medium/heavy gasoline truck engine market.
That being said, it ( BD 282 308) did last in one form or another almost as long. It was still an option in the late 60's Loadstar line, although few got them.